1. Field of the Invention
This invention relates generally to multi-stage optical amplifiers, and more particularly to broadband communication systems that include one or more multi-stage optical amplifiers.
2. Description of the Related Art
The demand for bandwidth continues to grow exponentially on fiber-optic superhighways due to applications such as data communications and the internet. Consequently, there is much effort at exploiting the bandwidth of optical fibers by using higher speeds per channel. Examples include time-division multiplexed systems-and wavelength-division multiplexing (WDM).
Most fiber-optic networks currently deployed use standard single-mode fiber or dispersion-shifted fiber (DSF). Standard fiber has a zero dispersion wavelength around 1310 nm, and the dispersion is primarily resulting from the inherent glass dispersion. Currently, most of the terrestrial network in the US and the world is based on standard fiber.
With DSF, waveguide dispersion is used to shift the zero dispersion wavelength to longer wavelengths. A conventional DSF has a zero dispersion wavelength at 1550 nm, coinciding with the minimum loss in a fused silica fiber. However, the zero dispersion wavelength can be shifted around by varying the amount of waveguide dispersion added. DSF is used exclusively in two countries, Japan and Italy, as well as in new long-haul links.
The limiting factors for a fiber-optic transmission line include loss, dispersion and gain equalization. Loss refers to the fact that the signal attenuates as it travels in a fiber due to intrinsic scattering, absorption and other extrinsic effects such as defects. Optical amplifiers can be used to compensate for the loss. Dispersion means that different frequencies of light travel at different speeds, and it comes from both the material properties and waveguiding effects. When using multi-wavelength systems and due the non-uniformity of the gain with frequency, gain equalization is required to even out the gain over the different wavelength channels.
The typical solution to overcoming these limitations is to periodically place in a transmission system elements to compensate for each of these problems. For example, a dispersion compensator can be used to cancel the dispersion, an optical amplifier used to balance the loss and a gain equalization element used to flatten the gain. Examples of dispersion compensators include chirped fiber gratings and dispersion compensating fiber (DCF). Examples of optical amplifiers include erbium-doped fiber amplifiers (EDFAs), Raman amplifiers, and non-linear fiber amplifiers (NLFAs).
Another problem that arises in WDM systems is interaction or cross-talk between channels through non-linearities in the fiber. In particular, four-wave mixing (4WM) causes exchange of energy between different wavelength channels, but 4WM only phase matches near the zero dispersion wavelength. Consequently, if a fiber link is made from conventional DSF, it is difficult to operate a WDM system from around 1540-1560 nm. This turns out to be quite unfortunate because typical EDFA""s have gain from 1535-1565 nm, and the more uniform gain band is near 1540-1560 nm. A second fiber nonlinearity that can be troublesome is modulation instability (MI), which is 4WM where the fiber""s nonlinear index-of-refraction helps to phase match. However, MI only phase matches when the dispersion is positive or in the so-called soliton regime. Therefore, MI can be avoided by operating at wavelengths shorter than the zero dispersion wavelength.
As the bandwidth utilization over individual fibers increases, the number of bands used for transmission increases. For WDM systems using a number of bands, additional complexities arise due to interaction between and amplification in multi-band scenarios. In particular, particular system designs are needed for Raman amplification in multi-band transmission systems. First, a new nonlinearity penalty arises from the gain tilt from the Raman effect between channels. This arises because long wavelength channels tend to rob energy from the short wavelength channels. Therefore, a means of minimizing the gain tilt on existing channels with the addition of new WDM channels is required.
To minimize both the effects of 4WM and Raman gain tilt, another technical strategy is to use distributed Raman amplification. In a WDM system with multi-bands, a complexity arises from interaction between the different pumps along the transmission line.
There is a need for greater bandwidth for broadband communication systems. A further need exists for broadband communication systems with reduced loss. Yet another need exists for broadband communication systems in the short wavelength region (S-band) covering the wavelength range of approximately 1430-1530 nm. Another need exists for broadband communication systems with improved dispersion compensation.
Accordingly, it is an object of the present invention to provide improved multi-stage optical amplifiers and broadband communication systems.
Another object of the present invention is to provide multi-stage optical amplifiers and broadband communication systems with greater bandwidth.
Yet another object of the present invention is to provide multi-stage optical amplifiers and broadband communication systems in the S band.
A further object of the present invention is to provide multi-stage optical amplifiers and broadband communication systems that use standard fiber and DSF with different zero dispersion wavelengths.
Another object of the present invention is to provide a multi-stage optical amplifier and broadband communication system that combines the C and S bands.
Yet another object of the present invention is to provide multi-stage optical amplifiers and broadband communication systems that combine the C, S and L bands.
A further object of the present invention is to provide multi-stage optical amplifiers and broadband communication systems with gain tilt control.
It is yet another object of the present invention to provide WDM systems over DSF links by using the xe2x80x9cvioletxe2x80x9d band in Raman amplifiers with dispersion compensating fiber to avoid nonlinearity limitations from 4WM and MI.
These and other objects of the present invention are achieved in a multi-stage optical amplifier that has an optical fiber. The optical fiber includes at least a first Raman amplifier fiber and a second Raman amplifier fiber. The optical fiber is configured to be coupled to at least one signal source that produces at least a signal wavelength xcexs and at least two pump sources that collectively produce a pump beam of wavelength xcexp. Pump wavelength xcexp is less than signal wavelength xcexs. Signal input, signal output and a first pump input port are each coupled to the optical fiber. The first Raman amplifier fiber is positioned between the signal input port and the pump input port. The second Raman amplifier fiber is positioned between the pump input port and signal output port. A second pump input port is coupled to the optical fiber and positioned between the second Raman amplifier fiber and the signal output port. A first lossy member is positioned between the pump input port and the signal output port. The lossy member is lossy in at least one direction so that passage of the pump radiation of wavelength xcexp from the second to the first length of amplifier fiber is substantially blocked. The signal flows in a first direction and the pump beam flows in a reverse direction relative to the first direction.
In another embodiment, the present invention is a broadband communication system with a transmitter and a receiver. An optical fiber is coupled to the transmitter and receiver. The optical fiber includes at least a first Raman amplifier fiber and a second Raman amplifier fiber. The optical fiber is configured to be coupled to at least one signal source that produces at least a signal wavelength xcexs and at least two pump sources that collectively produce a pump beam of wavelength xcexp. Pump wavelength xcexp is less than signal wavelength xcexs. Signal input, signal output and a first pump input port are each coupled to the optical fiber. The first Raman amplifier fiber is positioned between the signal input port and the pump input port. The second Raman amplifier fiber is positioned between the pump input port and signal output port. A second pump input port is coupled to the optical fiber and positioned between the second Raman amplifier fiber and the signal output port. A first lossy member is positioned between the pump input port and the signal output port. The lossy member is lossy in at least one direction so that passage of the pump radiation of wavelength xcexp from the second to the first length of amplifier fiber is substantially blocked.